Krita - Krita is a professional FREE and open source painting program. It is made by artists that want to seaffordable art tools for everyone. Concept art. texture and matte painters, illustrations and comics.
GIMP - GIMP is a multiplatform photo manipulation tool.
Artrage - A simple but lovely natural media painting and sketching program.
Paint.NET - Paint.NET is a free image and photo editing software.
PaintTool SAI - Learn to draw in different ways.
TwistedBrush Pro Studio - A powerful and unique natural media painting program.